青海鄂拉山北段牦牛溝銅金礦輝鉬礦Re-Os年齡及其地質(zhì)意義

陳永福, 張　棟, 路英川, 劉　鵬, 王曉軍, 傅　揚, 李文良, 郭曉東

中國人民武裝警察部隊黃金地質(zhì)研究所, 河北廊坊 065000

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青海鄂拉山北段牦牛溝銅金礦輝鉬礦Re-Os年齡及其地質(zhì)意義

陳永福, 張棟, 路英川, 劉鵬, 王曉軍, 傅揚, 李文良, 郭曉東

中國人民武裝警察部隊黃金地質(zhì)研究所, 河北廊坊 065000

摘要:鄂拉山是中央造山帶西段之東昆侖和西秦嶺的結(jié)合部位, 由于研究程度不高而影響著大家對其區(qū)域印支期成礦作用、成礦地質(zhì)背景及地球動力學(xué)過程的認識。因此, 本文從特定礦床類型時空分布特征反演成礦地質(zhì)背景和地球動力學(xué)過程的方法入手, 以鄂拉山北段的牦牛溝銅金礦為研究對象, 詳細研究其礦床地質(zhì)特征和成礦年代學(xué), 將其與區(qū)域東昆侖典型斑巖-矽卡巖型礦床進行時空對比研究, 并結(jié)合傳統(tǒng)區(qū)域大地構(gòu)造研究和成巖證據(jù), 試圖探討區(qū)域成礦地質(zhì)背景、地球動力學(xué)過程以及鄂拉山的歸屬問題。結(jié)果表明, 牦牛溝銅金成礦為小型矽卡巖礦床, 成礦與木勒兒花崗閃長巖關(guān)系密切。與銅金成礦有關(guān)的礦化主要為硅化、絹云母化、黃銅礦化和黃鐵礦化。礦床礦石礦物有黃銅礦、黃鐵礦以及輝鉬礦等。首次獲得有意義的輝鉬礦Re-Os加權(quán)平均年齡為(238.7±1.4) Ma。同時, 三疊紀鄂拉山與東昆侖具有相似的地質(zhì)演化特征,屬于東昆侖南北向擠壓構(gòu)造背景下的成礦事件。東昆侖三疊紀成礦可能有兩個相對峰期: 約237 Ma以矽卡巖礦床為主的鐵銅成礦期和約222 Ma的以斑巖為主的銅金成礦期, 前者可能和俯沖碰撞轉(zhuǎn)換期的板塊斷離有關(guān), 后者則與后碰撞拆沉背景有關(guān)。

關(guān)鍵詞:輝鉬礦Re-Os年齡; 牦牛溝; 鄂拉山; 東昆侖

本文由中國地質(zhì)調(diào)查局地質(zhì)調(diào)查項目“青海省秦嶺西段金多金屬礦產(chǎn)成礦規(guī)律與找礦預(yù)測研究”(編號: 1212011220899)資助。收稿日期: 2015-04-26; 改回日期: 2015-08-09。責(zé)任編輯: 閆立娟。

受成礦年代學(xué)研究的制約, 早期研究(Sawkins, 1990)認為特定礦床類型與其成礦大地構(gòu)造環(huán)境具有一定的關(guān)系(Kerrich et al., 2005)。近來研究表明成礦系統(tǒng)是地球動力學(xué)過程的有機組成, 特定礦床類型的時空分布可以約束其產(chǎn)出的大地構(gòu)造環(huán)境,并可與傳統(tǒng)大地構(gòu)造研究和成巖證據(jù)共同約束區(qū)域地球動力學(xué)演化(Groves et al., 2007; Pirajno, 2009)。因此, 精確的成礦年代學(xué)研究對理解成礦作用及其地質(zhì)演化至關(guān)重要。而輝鉬礦Re-Os定年體系由于其高濃度的Re和缺少普通Os而被認為是有效的成礦年代學(xué)方法之一(Stein et al., 1997)。

鄂拉山是我國重要銅多金屬成礦帶之一(宋治杰等, 1995), 區(qū)域分布著眾多矽卡巖礦床。其中,北段以小型銅金礦為主, 成礦時代初步確定為晚三疊世(青海省地質(zhì)三隊, 1985); 而南段成礦年齡集中于223 Ma左右(劉建平等, 2012; 王輝等, 2015),且部分礦床具有斑巖成礦潛力(李東生等, 2009)。然而, 有限的年代學(xué)研究使得學(xué)者對鄂拉山三疊紀地質(zhì)演化的認識存在較大差異, 部分學(xué)者(Chen et al., 2012; 楊立功等, 2013)認為鄂拉山作為東昆侖的一部分是布青山—阿尼瑪卿洋向北俯沖碰撞的結(jié)果;部分學(xué)者(孫延貴等, 2001, 2004)則認為是揚子板塊或西秦嶺有限向西斜向俯沖的結(jié)果。而斑巖成礦系統(tǒng)(包括斑巖和矽卡巖礦床)常形成于造山環(huán)境, 多見于俯沖或后碰撞階段(Meinert et al., 2005; Richards, 2009; Hou et al., 2012), 是有效的大地構(gòu)造環(huán)境指標(biāo)。本文從鄂拉山北段牦牛溝矽卡巖礦床的精確輝鉬礦Re-Os年代學(xué)研究入手, 綜合對比區(qū)域東昆侖和鄂拉山成礦年代學(xué)數(shù)據(jù)并輔以少量成巖證據(jù), 試圖探討三疊紀鄂拉山區(qū)域歸屬及地球動力學(xué)演化過程。

東昆侖造山帶(圖1a)是我國中央造山帶的重要組成部分(Pan et al., 2012), 是青藏高原內(nèi)可與岡底斯相媲美的一條巨型構(gòu)造巖漿巖帶(莫宣學(xué)等, 2007)。帶內(nèi)花崗巖形成可劃分為前寒武紀、早古生代(如, 李瑞保等, 2014)、晚古生代(如, 劉彬等, 2012; 陸露等, 2013)—早中生代和晚中生代—新生代四個時段。其中尤以三疊紀花崗巖最為發(fā)育(莫宣學(xué)等, 2007), 且240 Ma的幔源巖漿底侵和巖漿混合(劉成東等, 2004; Chen et al., 2012)作用標(biāo)志著俯沖碰撞轉(zhuǎn)換時期(莫宣學(xué)等, 2007; 張智勇, 2008)。

鄂拉山處于秦祁昆復(fù)合部位東昆侖東段(圖1a),同時也屬于東昆侖造山帶六個次級構(gòu)造巖漿帶之一的宗務(wù)隆—鄂拉山構(gòu)造巖漿帶(Pan et al., 2012)。在晚古生代時期, 鄂拉山構(gòu)造巖漿帶沿宗務(wù)隆山—同仁隆務(wù)峽及以南的部分地區(qū), 受后碰撞地殼伸展影響形成裂陷盆地或小洋盆, 從而構(gòu)成布青山—阿尼瑪卿古特提斯洋的分支洋(張智勇等, 2004), 向南可能與阿尼瑪卿裂陷連通構(gòu)成三向連結(jié)構(gòu)造, 其中北支為苦?！愂蔡辽呔G混雜巖帶, 其北帶缺乏陸緣弧火山巖帶和蛇綠混雜巖帶, 僅有前寒武紀裂解巖塊和石炭紀—二疊紀淺海碳酸鹽巖塊組成的疊置帶(孫延貴等, 2004)。早、中三疊世盆地裂陷達高峰, 接受了五千至萬余米厚的砂巖、板巖組成的類復(fù)理石沉積; 中下三疊統(tǒng)最厚逾兩萬米, 含多層滑塌堆積和含早二疊世的外來灰?guī)r塊, 并在中三疊統(tǒng)上部出現(xiàn)變基性凝灰?guī)r、凝灰質(zhì)砂巖、流紋質(zhì)火山角礫凝灰?guī)r、凝灰質(zhì)砂板巖夾流紋巖等火山—沉積巖(宋治杰等, 1995)?；鹕綆r年齡集中在210~240 Ma, 為鈣堿性系列的I型巖漿, 屬造山帶火山巖(孫延貴等, 2001)。

東昆侖區(qū)域成礦主要集中在三疊紀中晚期(豐成友等, 2009b; 吳建輝等, 2010), 以陸陸碰撞或陸內(nèi)俯沖有關(guān)的造山型金礦和矽卡巖型鐵多金屬礦床為主(豐成友等, 2004), 并呈現(xiàn)出斑巖型成礦帶的雛形(豐成友等, 2009b)。而鄂拉山成礦帶則呈北西—南東展布(宋治杰等, 1995), 南段產(chǎn)出賽什塘、銅峪溝、索拉溝及日龍溝等中大型銅、錫多金屬礦床; 北段有牦牛溝銅金礦以及沙龍瓜孔、占順南、占順北、都休瑪、阿尕澤等小型銅、鐵礦床(點)(圖1b)。礦體多沿花崗閃長巖巖體內(nèi)外接觸帶發(fā)育, 均產(chǎn)于矽卡巖中(青海省地質(zhì)三隊, 1985)。

圖1　阿爾金—祁連—昆侖俯沖增生山鏈平面圖(a; 許志琴等, 2006)和牦牛溝銅金礦區(qū)域地質(zhì)圖(b; 根據(jù)青海省地質(zhì)三隊, 1985修改)Fig. 1　Paleozoic Altun–Qilian–Kunlun mountain chains of subduction accretional type in the western part of the Central Orogenic Belt (a; after XU et al., 2006) and Regional geological map of the Maoniugou Cu-Au skarn ore deposit (b; modified after Qinghai No. 3 Geological Party, 1985)TRMB-塔里木地塊; NCB-華北板塊; BYSG+QT-巴彥喀拉—松潘甘孜地塊+羌塘地塊; NQLS-北祁連俯沖雜巖帶; QL-祁連地塊; SQLF-南祁連走滑斷裂; NQDMS-柴北緣俯沖雜巖帶; GDM-柴達木地塊; QMTGS-祁漫塔格俯沖雜巖帶; N.EKL-東昆北地塊; CKLS-昆中俯沖雜巖帶; S.EKL-東昆南地塊; EKLF-東昆侖走滑斷裂; ANMQS-阿尼瑪卿俯沖雜巖帶; ALTF-阿爾金走滑斷裂; ALT-阿爾金地體TRMB-Tarim Block; NCB-North China Block; BYSG+QT-Bayan Har–Songpan-Ganzi and Qiangtang terranes; NQLS-North Qilian Subduction Complex Belt; QL-Qilian Terrane; SQLF-South Qilian Strike-slip Fault; NQDMS-Northern Margin of Qaidam Subduction Complex Belt; GDM-Qaidam Terrane; QMTGS-Qimantag Subduction Complex Belt; N.EKL-Northern East Kunlun Terrane; CKLS-CentralEast Kunlun Subduction Complex Belt; S.EKL-Southern East Kunlun Terrane; EKLF-East Kunlun Strike-slip Fault; ANMQS-Animaqin Subduction Complex; ALTF-Altun Strike-slip Fault; ALT-Altun terrane

1　礦床地質(zhì)特征

牦牛溝(又名澤很錯)銅金礦是鄂拉山多金屬成礦帶北段的一個小型矽卡巖型礦床, 位于木勒爾花崗閃長巖體(γδ51b)北東側(cè)外接觸帶(青海省地質(zhì)三隊, 1985), 礦床距巖體約3 km(圖1b)。礦床容礦圍巖為中三疊統(tǒng)古浪提群a組板巖段(T2gla-2), 該段地層自下而上分別為黑云母巖化板巖、變泥巖以及變泥巖與長石石英砂巖互層。其中變泥巖下部夾似層狀、透鏡狀長石石英砂巖, 并見有透鏡狀、不規(guī)則狀大理巖零星分布, 銅金礦體產(chǎn)在該層中、下部(圖2)。木勒爾巖體北東部花崗閃長巖與圍巖地層呈“港灣”狀接觸, 頂面起伏大, 總體傾向北東, 并產(chǎn)生北東向?qū)捈s3 km的黑云母角巖化帶?；◢忛W長巖具中細粒自形粒狀結(jié)構(gòu)、局部呈似斑狀結(jié)構(gòu), 塊狀構(gòu)造。巖石的礦物成分為斜長石(45%~60%)、鉀長石(10%~20%)、石英(15%~20%)、角閃石(5%~8%)、黑云母(3%~7%), 局部含有輝石。副礦物有磁鐵礦、鋯石、磷灰石、金紅石、榍石等。SiO2含量變化小,介于61.79%~67.86%, 全堿含量較高, Na2O+K2O=6.47%~7.16%, 屬鈣堿性巖石系列(青海省地質(zhì)三隊, 1985)。

圖2　牦牛溝金礦床地質(zhì)簡圖(據(jù)青海省地質(zhì)三隊, 1985修改)Fig. 2　Sketch geological map of the Maoniugou gold ore deposit(modified after Qinghai No. 3 Geological Party, 1985)

牦牛溝銅金礦主要的礦體為ICu Au、IIAu和IIIAu(圖2), 均與矽卡巖相伴產(chǎn)出。由于受近東西向和北東向接觸帶構(gòu)造控制, 三個礦體分別呈不規(guī)則脈狀、囊狀和透鏡狀。礦體走向近東西或北東, 傾向近南或南東, 傾角54o~68o。礦體斜切圍巖或者總體與圍巖產(chǎn)狀一致, 與圍巖呈不規(guī)則狀接觸。其中, I號Cu Au礦體較為典型, 長28.7 m, 厚0.16~1.54 m, 平均厚度為0.54 m; 品位較富, Cu為0.38%~12.43%; Au為8~22 g·t-1, 平均12.06 g·t-1(青海省地質(zhì)三隊, 1985)。與銅金成礦有關(guān)的礦石礦物有黃銅礦、黃鐵礦、輝鉬礦等, 呈星點狀或細脈狀分布。金在矽卡巖中呈自然金產(chǎn)出, 其形態(tài)有樹枝狀、棒狀、片狀、粒狀、板狀等(青海省地質(zhì)三隊, 1985)。

與銅金成礦有關(guān)的圍巖蝕變主要為矽卡巖化,其次是硅化、絹云母化。矽卡巖化主要包括石榴石矽卡巖, 透輝石石榴石矽卡巖和透輝石矽卡巖。主要的矽卡巖礦物有石榴石、透輝石、符山石、陽起石、綠泥石、絹云母、石英、方解石等。牦牛溝銅金礦蝕變礦化可分為矽卡巖期和熱液期, 其中熱液期分為石英-硫化物階段(主要形成金屬硫化物、綠泥石、絹云母、石英和方解石)和碳酸鹽階段(形成大量方解石和少量石英等)。石英-硫化物階段為主成礦期。

2　輝鉬礦Re-Os同位素年齡測定

6個輝鉬礦Re-Os年齡樣品均采自牦牛溝銅金礦(經(jīng)緯度為: 36o05'17"N; 99o37'52"E)礦體(圖2), 樣品為黃銅礦化、輝鉬礦化含矽卡巖礦石, 其中輝鉬礦以浸染狀和團塊狀產(chǎn)出(圖3)。樣品分布均勻, 具有較強的代表性。輝鉬礦均采用人工破碎重砂淘洗法進行分離, 并在雙目鏡下挑純, 純度大于99%, 晶體無氧化、無污染。輝鉬礦的Re、Os同位素測試在中國地質(zhì)科學(xué)院國家地質(zhì)實驗測試中心進行, 采用美國TJA公司生產(chǎn)的電感耦合等離子體質(zhì)譜儀TJA X-series ICP-MS測定同位素比值。實驗原理和分析流程參見文獻(屈文俊和杜安道, 2003; Du et al., 2004)。輝鉬礦標(biāo)準物質(zhì)為GBW04436 (JDC), 本次試驗測得平均年齡為(141.0±2.2) Ma, 與標(biāo)準物質(zhì)標(biāo)準年齡值(139.6±3.8) Ma(Du et al., 2004)相比在誤差范圍內(nèi)。測定空白顯示Re為1.2 pg, Os為0.4 pg。

牦牛溝銅金礦輝鉬礦Re-Os同位素測定數(shù)據(jù)見表1, 6件輝鉬礦中Re的含量較高且比較接近, 水平為(239.7±8.0)~(180.0±1.5) μg·g-1, 總體187Re與1 8 7O s含量比較協(xié)調(diào), 輝鉬礦模式年齡介于(237.2±3.3) ~(240.7±3.3) Ma之間, 加權(quán)平均年齡為(238.7±1.4) Ma(圖4)。而等時線年齡為(248±16) Ma(圖4), 等時線與187Os軸的截距在不確定度范圍內(nèi)接近于零, 說明187Os不含放射性成因成分。表明輝鉬礦Re-Os年齡是可靠的, 可以用t = (1/λ)ln(1+187Os/187Re)方程計算(Stein et al., 1997),但等時性年齡誤差相對較大。

表1　牦牛溝矽卡巖型銅金礦中輝鉬礦Re-Os同位素數(shù)據(jù)Table 1　Re-Os isotopic data for molybdenite from the Maoniugou skarn Cu-Au ore deposit

圖3　牦牛溝銅金礦床礦石和部分測年樣品特征Fig. 3　Features of ores and dating samples of the Maoniugou Cu-Au ore deposita-石榴石矽卡巖; b, c, d-輝鉬礦年齡樣品; Mo-輝鉬礦; Ccp-黃銅礦; Cal-方解石; Grt-石榴石a-garnet skarn; b, c, d-molybdenite dating sample; Mo-molybdenite; Ccp-chalcopyrite; Cal-calcite; Grt-garnet

3　成礦年代學(xué)及其地質(zhì)意義

鄂拉山多金屬成礦帶北段雖然礦床(點)眾多,但至今沒有可靠的成礦年代學(xué)數(shù)據(jù), 這極大限制了區(qū)域成礦作用及地球動力學(xué)過程研究。本次研究顯示輝鉬礦Re-Os年齡數(shù)據(jù)沒有初始和普通Os, 其輝鉬礦等時線年齡為(248±16) Ma, 誤差較大; 而模式年齡非常一致, 其加權(quán)平均年齡為(238.7±1.4) Ma,為有意義的成礦年齡數(shù)據(jù)。且礦物學(xué)研究表明, 輝鉬礦與黃銅礦、黃鐵礦以及矽卡巖共生, 其年齡可代表礦床物質(zhì)沉淀的時間。

圖4　牦牛溝銅金礦輝鉬礦Re-Os等時線模式年齡和加權(quán)平均年齡Fig. 4　Re-Os isochron diagram and weighted average model age diagram for Maoniugou Cu-Au ore deposit

以牦牛溝銅金礦為代表的鄂拉山北段礦床成礦年齡為238.7 Ma, 而以賽什塘銅礦床為代表的鄂拉山南段成礦年齡約為223 Ma(劉建平等, 2012; 王輝等, 2015), 這預(yù)示著鄂拉山多金屬成礦帶三疊紀可能存在兩期成礦事件或者同期成礦事件的不同成礦階段。資料表明東昆侖西段祁漫塔格成礦帶具有同樣的特征, 中晚三疊世與花崗巖有關(guān)的矽卡巖和斑巖型成礦具有兩個峰期, 且矽卡巖成礦早于斑巖成礦約14 Ma(豐成友等, 2010)?；谝陨戏治? 筆者認為鄂拉山和東昆侖可能具有相同的成礦動力學(xué)過程。因此, 對區(qū)域東昆侖和鄂拉山較為典型的斑巖型和矽卡巖型礦床成礦年代學(xué)和礦床時空分布進行分析。結(jié)果(表2; 圖5)顯示, 區(qū)域成礦年齡統(tǒng)計結(jié)果相似, 均落在244.2~234.5 Ma和224.7~213 Ma兩個范圍。其中, 前者以矽卡巖型礦床為主, 后者以斑巖型礦床為主。對相關(guān)數(shù)據(jù)合并統(tǒng)計后, 顯示兩個峰期年齡分別為約237 Ma和約222 Ma(圖6),相差約17 Ma。另外, 與牦牛溝銅金礦有關(guān)的木勒爾巖體花崗閃長巖巖石化學(xué)數(shù)據(jù)顯示活動陸緣的特征(青海省地質(zhì)三隊, 1985)。共同表明鄂拉山和東昆侖可能具有相似的成礦作用和構(gòu)造背景(俯沖或者碰撞環(huán)境)。

結(jié)合傳統(tǒng)大地構(gòu)造研究對約束區(qū)域地球動力學(xué)過程(Groves et al., 2007; Pirajno, 2009)將有極大幫助。在鄂拉山, 共和盆地周緣存在兩期清晰的構(gòu)造巖漿事件, (235±2) Ma的黑馬河巖體巖漿事件和(218±2) Ma的溫泉巖體巖漿事件(張宏飛等, 2006)。相鄰的東昆侖東部約格魯花崗閃長巖((242±6) Ma)和角閃輝長巖((239±6) Ma)鋯石U-Pb年齡顯示相近年齡的巖漿事件(劉成東等, 2004)。對于東昆侖地區(qū),其北側(cè)宗務(wù)隆帶南側(cè)發(fā)育的246 Ma天駿南山花崗巖、238 Ma青海湖南山花崗巖和215 Ma二郎洞花崗巖(郭安林等, 2009), 中東部香日德以北的244 Ma花崗巖類成巖峰期(Chen et al., 2012)和244 Ma的阿斯哈閃長巖鋯石U-Pb年齡(李碧樂等, 2012)以及香日德地區(qū)的223 Ma的二長花崗巖年齡和220 Ma的花崗閃長巖年齡(羅明非等, 2014), 還有東昆侖山中部的241~236 Ma花崗巖類年齡(張雪亭等, 2005)以及東昆侖西部祁漫塔格地區(qū)230~ 237 Ma的發(fā)育閃長質(zhì)暗色微粒包體的中三疊世花崗巖和形成于204~228 Ma的具有斑狀或似斑狀結(jié)構(gòu)的晚三疊世高分異富鉀花崗巖事件(豐成友等, 2012)同樣顯示東昆侖也存在著相似的兩期巖漿事件。另外, 對比中央造山帶的秦嶺造山帶三疊世花崗巖漿活動表明, 也具有兩個相對峰期: 分別為235~225 Ma的小規(guī)模石英閃長巖和220~210 Ma左

右的大規(guī)模高鉀鈣堿性花崗巖(秦江鋒, 2010)。陳衍景(2010)也對秦嶺印支期構(gòu)造背景、巖漿活動及成礦作用進行了詳細的總結(jié)(在此不做詳細討論), 表明秦嶺地區(qū)在245~200 Ma期間發(fā)生了一次強烈的成礦作用。總體相對晚于東昆侖。這些不完全統(tǒng)計的巖體年齡說明, 包括鄂拉山在內(nèi)的東昆侖以及秦嶺區(qū)域地球動力演化過程中, 確實存在著兩期和矽卡巖、斑巖成礦有關(guān)的巖漿活動。這表明代表第一成礦峰期的237 Ma左右, 東昆侖區(qū)域演化至少發(fā)展為俯沖或俯沖后環(huán)境。而區(qū)域沿柴達木微陸塊分布的264 Ma左右的早二疊世弧火山巖和267 Ma的納木龍俯沖型花崗巖表明該時期存在著苦?！愂蔡练种а蟮南虮备_消減(張智勇等, 2004)。同樣,東昆侖祁漫塔格地區(qū)(284.3±1.2) Ma、(270.9± 0.9) Ma、(261.5±3.7) Ma和(257.1±0.7) Ma的花崗巖類年齡, 東昆侖東段?？拥吕账固?268.5±0.8) Ma的二長花崗巖鋯石年齡以及白日其利鎂鐵質(zhì)巖墻群的(251±2) Ma結(jié)晶侵位年齡都表明東昆侖在251 Ma之前仍處于俯沖背景下的大陸邊緣弧環(huán)境(王秉璋等, 2009; 熊富浩等, 2011; 楊延乾等, 2013; Li et al., 2013)。而260~230 Ma, 東昆侖已處于俯沖向碰撞轉(zhuǎn)換階段(羅照華等, 2002; 張智勇, 2008; Huang et al., 2014)。那么, 東昆侖矽卡巖、斑巖成礦的兩個峰期(約237 Ma和約222 Ma)顯然處于俯沖后環(huán)境。

表2　東昆侖區(qū)域成礦典型礦床年代學(xué)Table 2　Mineralization age of representative ore deposits in East Kunlun

圖5　東昆侖區(qū)域斑巖-矽卡巖型礦床分布及成礦年代學(xué)(數(shù)據(jù)見表2, 底圖據(jù)潘桂堂, 2004)Fig. 5　Distribution and ore-forming geochronology of porphyry-skarn ore deposits in East Kunlun (for data see Table 2, base map after PAN, 2004)

圖6　東昆侖區(qū)域斑巖-矽卡巖型成礦年代學(xué)直方圖(數(shù)據(jù)見表2)Fig. 6　Histogram for mineralization age of porphyry-skarn ore deposits in East Kunlun (for data see Table 2)

東昆侖多期復(fù)合造山?jīng)Q定著區(qū)內(nèi)斑巖-矽卡巖型礦床的時空分布(吳建輝等, 2010)。而區(qū)域大地構(gòu)造研究表明, 包括鄂拉山在內(nèi)的東昆侖成礦具有俯沖后成礦特征, 約237 Ma的成礦峰期可能與俯沖結(jié)束碰撞開始時的板片斷離作用有關(guān), 板片斷離作用導(dǎo)致軟流圈物質(zhì)上隆并誘發(fā)地幔楔的減壓熔融,產(chǎn)生鎂鐵質(zhì)巖漿, 在下地殼底部造成底侵作用(羅照華等, 2002; Huang et al., 2014), 形成矽卡巖成礦為主的鐵銅礦床; 而約222 Ma的成礦峰期則可能與巖石圈拆沉背景有關(guān)(張宏飛等, 2006), 主要發(fā)育斑巖型銅金礦床。但最新找礦趨勢看, 東昆侖區(qū)域具有斑巖型礦床的優(yōu)越地質(zhì)條件(豐成友等, 2009b),區(qū)域斑巖-矽卡巖系統(tǒng)型礦床找礦潛力很大。

4　結(jié)論

牦牛溝礦床是東昆侖東段鄂拉山北部的一個小型矽卡巖型銅金礦床。其輝鉬礦等時線年齡為(248±16) Ma, 誤差較大; 而模式年齡非常一致, 其加權(quán)平均年齡為(238.7±1.4) Ma, 為有意義的成礦年齡。

東昆侖和鄂拉山地區(qū)發(fā)育相近的約237 Ma和約222 Ma的兩個相對成礦峰期, 早期以矽卡巖型成礦為主, 而晚期以斑巖成礦為主。區(qū)域斑巖-矽卡巖系統(tǒng)型礦床找礦潛力不可忽視。

東昆侖和鄂拉山三疊紀中晚期可能處于相同的俯沖后構(gòu)造環(huán)境并具有相似的地球動力學(xué)過程,同受布青山—阿尼瑪卿洋向北俯沖作用控制。260~230 Ma由俯沖向碰撞轉(zhuǎn)換及其后, 發(fā)育與兩個成礦峰期有關(guān)的巖漿活動, 前者可能與俯沖碰撞轉(zhuǎn)換期的板塊斷離有關(guān), 后者與后碰撞拆沉背景有關(guān)。鄂拉山應(yīng)是東昆侖的一部分。

Acknowledgements:

This study was supported by China Geological Survey (No. 1212011220899).

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Re-Os Dating of Molybenite from the Maoniugou Cu-Au Ore Deposit in the Northern Part of the Ngola Mountain, Qinghai Province, and Its Geological Significance

CHEN Yong-fu, ZHANG Dong, LU Ying-chuan, LIU Peng, WANG Xiao-jun, FU Yang, LI Wen-liang, GUO Xiao-dong
Institute of Gold Geology, Chinese Armed Police Force, Langfang, Hebei 065000

Abstract:The Ngola Mountain lies at the conjunction between West Qinling and East Kunlun in the west of Central Orogenic Belt. Owing to its low research level, some problems concerned, such as the regional metallogenesis, geological setting and geodynamic process in Indosinian period, remain unclear. In this study, the authors used the method of temporal and spatial distribution characteristics of specific type deposits to deduce the geological background and geodynamic process. Based on a detailed study of geological features and ore-forming geochronology of the Maoniugou copper-gold deposit in northern Ngola Mountain, and combined with the traditional tectonic study and diagenetic evidence, the authors tried to explore the regional ore-forming geological background, the geodynamic process and the assignment of the Ngola Mountain through a comparison with the typical porphyry-skarn deposits in East Kunlun. The results show that the Maoniugou deposit is a small copper-gold skarn deposit closely related to Muleer granodiorite. Mineralizations associated with Cu-Au mineralization are mainly silicification, sericitization, chalcopyritization and pyritization. Ore minerals arebook=70,ebook=73mainly composed of chalcopyrite, pyrite and molybdenite. The authors measured Re-Os isotopes of representative molybdenite-bearing samples from the main orebody, and the six yielded model ages range from (237±3.3) Ma to (240.7±3.3) Ma with a well-constrained weighted average model age of (238.7±1.4) Ma. Based on these new age data and ages of regional porphyry-skarn ore deposits, the authors have reached the conclusion that the Ngola Mountain and East Kunlun had similar post-subduction settings and geodynamic processes at least in middle and late Triassic, and they belonged to the continental margin settings with the north subduction of Buqing-Anymaqin Mountain. The East Kunlun Triassic mineralization may have two peak periods: Fe-Cu skarn mineralization at about 237 Ma and Cu-Au porphyry mineralization at about 222 Ma, related respectively to break-off of the subducted slab and lithospheric delamination after crustal thickening due to collision between continental plates in Triassic.

Key words:molybdenite Re-Os age; Maoniugou; Ngola Mountain; East Kunlun

作者簡介:第一 陳永福, 男, 1980年生。博士, 高級工程師。主要從事黃金地質(zhì)勘查。E-mail: cyf04@mails.ucas.ac.cn。

中圖分類號:P618.4; P597.2

文獻標(biāo)志碼:A

doi:10.3975/cagsb.2016.01.07